Merge &quotPlaceholder content direction should not affect INPUT/TEXTAREA direction.&quot to M52

      Skip -webkit-input-placeholder elements when we compute direction for dir=auto in
HTMLElement::directionality.

- Using <bdi> for placeholders doesn't work. We want to inherit placeholder direction
  from the parent element.
- Using unicode-bidi CSS property doesn't work. It doesn't affect dir=auto
  computation.

BUG=625739

Review-Url: https://codereview.chromium.org/2145823003
Cr-Commit-Position: refs/heads/master@{#405069}
(cherry picked from commit 55f27bb5c4f4f99a2885dc67bd6e448a464505ed)

Review URL: https://codereview.chromium.org/2147353002 .

Cr-Commit-Position: refs/branch-heads/2743@{#637}
Cr-Branched-From: 2b3ae3b8090361f8af5a611712fc1a5ab2de53cb-refs/heads/master@{#394939}